1. Field of the Invention
The present invention relates to a method for controlling reaction conditions and liquid transfer conditions in the preparation of bisphenol A by reacting phenol with acetone. More particularly, the invention relates to a preparation process of high-purity bisphenol A by controlling the reaction conditions in the reaction of phenol with acetone through the on-line estimation of a computer of the composition of the reaction mixture, the saturated solubility of bisphenol A and the slurry concentration of the adduct of bisphenol A with phenol (hereinafter refferred to as bisphenol A-phenol adduct) in the reactor, and further controlling the liquid transfer conditions of the resulting reaction mixture to the next step.
2. Description of the Related Art
There has been an increased demand for bisphenol as a material for engineering plastics in addition to polycarbonate resin and epoxy resin. Colorless and high purity bisphenol A is required for these uses.
In a process for preparing high purity bisphenol A, for example, phenol is reacted with acetone in the presence of an acid catalyst, and the reaction product is treated to remove the catalyst, water and a small amount of phenol. The residual liquid mixture is cooled to crystallize bisphenol A in the form of a phenol adduct, precipitated crystals are separated from the mother liquor, and phenol is removed from the bisphenol A adduct to recover bisphenol A.
Japanese Laid-Open Patent Hei 01-180843 has disclosed a process for reacting phenol and acetone in a continuous type reactor, completely reacting the reaction mixture which contains unreacted acetone in the presence of an hydrochloric acid catalyst in a batch type reactor so as to decrease unreacted acetone to substantially zero, and thereafter treating the reaction product in a purification step.
The process carries out the reaction to form minimum amounts of by-products and impurities, and makes a post-treatment step as simple as possible to prepare high-purity bisphenol A. However, no disclosure is found on the realtime analyzing and controlling method required for carrying out the process.
In cases of conducting the reaction through two steps composed of continuous and batch type reactors, an important and difficult technique is how to maintain the command on the reaction system which is continuously affected by the changing operational environment and disturbances.
As a practical problem, for example, when the reaction has progressed exceeding the set point in the first step in the continuous type reactor, bisphenol A-phenol adduct crystallizes in said reactor and it becomes difficult to transfer the reaction mixture to the second step in the batch type reactor. Further, when the heat of reaction is removed by an external heat exchanger, crystals of said adduct severely decrease the heat transfer efficiency of the heat exchanger and therefore increase the reaction temperature. As a result, by-products and impurities are formed in large amounts, which increased, lead to separation cost for these substances in the post treatment step, and result in a decrease of the purity of the product bisphenol A. In the worst case, operation of the whole production steps must be stopped.
On the other hand, when the set point in the first step reaction is low in order to avoid the above problems, the reaction time in the batch type reactor of the second step increases, requires scale up of said reactor and leads to a severe reduction of production efficiency. In order to solve these problems, it is important to prescribe suitable set points and to carry out rapid and adequate control on the rate of reaction so as to always coincide practical operating conditions with the set points.
In order to conduct adequate control, it is required to know the reaction rate in every second within a short time. The rate of reaction is affected by various factors such as the raw materials phenol and acetone, catalyst hydrochloric acid, formed bisphenol A and water, by-products and impurities. These substances form three phase equilibrium in vapor-liquid-solid and it is hence very difficult to always manage and control the rate of reaction.
After finishing the reaction in the second step batch type reactor, the resulting slurry of the reaction mixture is continuously transfered to a dehydrochlorination column in order to distill off hydrogen chloride and water from said reaction mixture. In the course of transfer, another problem develops. A buffer tank is usually employed for transferring the slurry from the batch type reactor to a continuous post treatment step. Discharge of the reaction mixture from the batch type reactor to the buffer tank is carried out intermittently at a constant interval in usual cases, at a different interval in some cases. The reaction mixture in the buffer tank is continuously fed to the dehydrochlorination column. The composition of the reaction product slurry and concentration of the slurry must be as constant as possible. Variation of these factors in the dehydrochlorination column disturbs the distribution concentration in the direction of height and causes a fluctuation in the amount of distillation. As a result, the internal pressure of the column changes and the slurry encounters increases in the variation of concentration distribution, thereby provides an increase in the residual hydrogen chloride in the liquid discharged from the column, gives adverse effects on the equipment in the next separation and purification step, and leads to serious deterioration of hue which is the most important characteristic of the product bisphenol A.
On the other hand, when an excess amount of phenol is always fed to the liquid transfer line, or an excess amount is distilled out of the dehydrochlorination column in order to prevent the above problems, phenol is distilled in excess with hydrogen chloride and water, and an increased amount of phenol circulates in the system. As a result, utility costs become too high, excess facility is required for preventing discharge of phenol out of the system, and hence productivity of bisphenol A is severely impaired.
In order to maintain the composition of the reaction mixture or the slurry concentration of bisphenol A-phenol adduct at a constant level, it is required to understand within a short time the material balance in view of the variation of said composition due to the reaction in the batch type reactor or the buffer tank, the amount of phenol required for washing the batch type reactor, and the amount of phenol required for purging the level gauge.
The rate of reaction in the batch type reactor and the buffer tank can only be estimated by considering the composition of a mixture which is fed to said batch type reactor at the start of the reaction, the reaction temperature which varies with time during the reaction, and the three phase equilibrium of vapor-liquid-solid formed by the reaction mixture. However, it is very difficult to always manage and control these factors.
Accordingly, the objects of the present invention are to solve the above problems in the preparation of bisphenol A from phenol and acetone and to provide a process for preparing high purity bisphenol A by conducting reaction control and liquid transfer control of the reaction mixture under stable conditions.